In the drilling and production of oil and gas wells, it is typical practice to use a retrievable string equipped with a bottom hole assembly (BHA) including a drill bit to remove formation ahead of the drill bit and thus form a wellbore. The drill string has an axial through bore which provides a fluid path through the drill string and BHA. Circulation of drilling fluid is achievable by return through the annulus between the drill string and the wellbore. Periodically, the drill string is removed from the wellbore and a wellbore casing string comprising lengths of tubular casing sections coupled together end-to-end is positioned in the drilled wellbore and cemented in place in order to stabilize the wellbore before extending the wellbore farther. A smaller dimension drill bit is then inserted through the cased wellbore, to drill through the formation below the cased portion, to thereby extend the depth of the wellbore. A smaller diameter casing is then installed in the extended portion of the wellbore and also cemented in place. These casing operations may be concluded by running in a liner.
A liner is a narrower bore section or string of casing quality tubular that is used to case an open hole below a previously set casing string. The liner extends from the bottom of the open hole and overlaps into the previously set casing string. The overlap ranges from 100 feet to 500 feet. Liners are usually suspended from the previously set casing string by means of a liner hanger/packer assembly. As with the previously set casing, the liner tubular string is cemented in place to create a bond between the tubular and the drilled wellbore surface in the formation. In cementing the liner, the cement slurry is pumped down to the liner, followed by a displacement fluid that forces the cement out into the annulus between the borehole wall and the liner, and into the overlap between the liner and the previously set casing string. As a consequence, there remains a cement “plug” in the lower casing above the top of the liner at least, and cement is within or around the polished bore receptacle at the top of the liner.
Any subsequent wellbore operation to complete the well will require further drilling operations to drill the cement to re-open the through bore. In doing so, at least two different sized drill bits are required to complete the well, which requires sequential run in and pull out of the drill string to make the change over.
Since a round trip, i.e. run in and subsequent pull out, can take in excess of 24 hours, the current procedure represents a significant usage of offshore rig time, which at costs of the order of US$1 million per day in a typical deep offshore well location represents a considerable financial commitment.
The disclosure will now be described by way of illustrative example with reference to the accompanying drawings.